He just happened to be really good at fighting. However, the Ep3 novel declares that he never mastered a particular Lightsaber fighting form. Mace Windu comments that this is why he was able to defeat uh.. you know, the wheezing four saber robot guy - because he wasn't mentally locked in to a specific form, and that dude killed jedi by learning their form and countering it. This does suggest that he wasn't trained as a warrior.

I remember reading that jedi practicing lightsaber combat intentionally did this so that they couldn't be predicatable, with that in mind he actually might be better at lightsaber combat than you think - even to probably fill the jedi knight stereotype. Furthermore the first time Qui-gon noticed Obi-wan was because he was the top in the class at lightsaber combat. Tbh I never really saw Obi Wan use any abilities besides the standard push/pull/jump/mindtrick.